Electrical devices generate and emit electro-magnetic radiation/fields (EMF's) into their immediate vicinity. Such radiation not only interferes with the performance of other nearby electrical devices, but also creates harmful biological effects in humans. Realizing the potential dangers of this radiation exposure, governments, universities and corporations have undertaken numerous studies to establish the causal links between EMF exposure and certain forms of cancer and other diseases, as well as to establish guidelines for safe exposure levels.
Due to the ever expanding use of personal computers in our society and their close proximity to their users, EMF emissions from computer video display units such as CRT's, LCD's and gas discharge displays have been extensively studied. These studies have led, in countries like Sweden and France, to legislation requiring certain EMF emitting devices such as video display units to limit radiation emissions in the vicinity of the user. Sweden, for example, now requires video display units to meet what is known as the MPR II standard for EMF emissions. Even in the absence of specific legislation, video display unit manufacturers are now voluntarily designing future units to meet or exceed the MPR II standard; however, the vast majority of video display units in worldwide use today and the majority of the ones currently being sold do not meet this standard.
Video display units are a major emitter of EMF's due their use of electron beams, electrostatic fields and/or magnetic fields that are used to display graphic images, information and data. For example, CRT's employ a deflection coil to magnetically deflect an electron beam (or raster) carrying the signal of the information or picture to be displayed toward the CRT's fluorescent screen. However, the magnetic field created by the deflection coil passes through the CRT's housing and screen, thereby creating harmful EMF emissions in the vicinity of the user.
Since the strength of a magnetic field decreases with distance, it has been suggested that the harmful biological effects of the magnetic field emitted from video display units could be avoided by maintaining an adequate, safe distance from such units, such as one meter. Also, as the harmful effects of magnetic fields increase with exposure, it has been suggested that the harmful effects can be reduced by turning off the video display unit when not in use.
Unfortunately, many video display units must be in use for substantial lengths of time making it unfeasible to reduce emissions by turning off the device. Further, due to the small character size displayed on most of such units, it is impractical to maintain an adequate safe distance from the unit.
In addition to the magnetic fields emitted from video display units, there exist ambient magnetic fields produced by the earth and other electrical devices. These ambient magnetic fields can interfere with the proper operation of the above-mentioned video display units by altering the color display and shifting the raster beam.
Numerous techniques have been developed to block the introduction of external ambient magnetic fields into video display units to prevent interference with their normal operation. However, such magnetic cancellation devices tend to increase the total magnetic field emitted from the unit, thereby increasing the interference with other devices in the vicinity and increasing the harmful biological effects caused by the magnetic field emissions. For example, U.S. Pat. No. 5,066,891 to Harrold et al. discloses a magnetic field sensing and cancelling circuit for use with a CRT which comprises a closed loop system of flux gate sensors to measure external magnetic fields and magnetic field cancellation coils disposed around the perimeter of the face plate of the CRT to cancel the measured field within the CRT housing. In this manner, color and positional image distortion of the CRT is minimized. The cancellation coils, however, create a net positive magnetic field outside of the CRT housing, thereby increasing the magnetic field emissions directed at the user.
What is needed is an apparatus and method to reduce, in the vicinity of the user, the intensity of the magnetic field emitted from video display units that can be easily retrofitted to existing units without distorting the unit's color and positional images.